Planet found orbiting habitable zone of sun-like star

Reuters, MOFFET FIELD, CALIFORNIA

A diagram compares our own solar system with Kepler-22, a star system containing the first “habitable zone” planet discovered by NASA’s Kepler mission, in this undated -handout picture released by NASA on Monday.

Photo: REUTERS/NASA

The most Earth-like planet ever discovered is circling a star 600 light years away, a key finding in an ongoing quest to learn if life exists beyond Earth, scientists said on Monday.

The planet, called Kepler-22b, joins a list of more than 500 planets found to orbit stars beyond our solar system. It is the smallest and the best positioned to have liquid water on its surface — among the ingredients necessary for life on Earth.

“We are homing in on the true Earth-sized, habitable planets,” said San Jose State University astronomer Natalie Batalha, deputy science team lead for NASA’s Kepler Space Telescope, which discovered the star.

The telescope, which was launched three years ago, is staring at about 150,000 stars in the constellations Cygnus and Lyra, looking for faint and periodic dimming as any circling planets pass by, relative to Kepler’s line of sight.

Results will be extrapolated to determine the percentage of stars in the Milky Way galaxy that harbor potentially habitable, Earth-size planets.

This is the first detection of a potentially habitable world orbiting a Sun-like star, scientists reported in findings to be published in The Astrophysical Journal.

Kepler-22b is 600 light years away. A light year is the distance light travels in a year, about 10 trillion kilometers.

Planets about the same distance from their parent stars as Earth take roughly a year to complete an orbit. Scientists want to see at least three transits to be able to rule out other explanations for fluctuations in a star’s light, such as small companion stars. Results are also verified by ground and other space telescopes.

Kepler-22b, which is about 2.4 times the radius of Earth, sits squarely in its star’s so-called “habitable zone,” the region where liquid water could exist on the surface. Follow-up studies are under way to determine if the planet is solid, like Earth, or more gaseous, like Neptune.

“We don’t know anything about the planets between Earth-size and Neptune-size because in our solar system we have no examples of such planets. We don’t know what fraction are going to be rocky, what fraction are going to be water worlds, what fraction are ice worlds. We have no idea until we measure one and see,” Batalha said at a news conference at NASA Ames Research Center in Moffet Field, California.

If Kepler-22b has a surface and a cushion of atmosphere similar to Earth’s, it would be about 22°C, about the same as a spring day in Earth’s temperate zone.

Among the 2,326 candidate planets found by the Kepler team, 10 are roughly Earth-sized and reside in their host stars’ habitable zones.